Method of adhering silicone rubber to metal surfaces using salts

ABSTRACT

The adhesion to metal surfaces of room temperature vulcanizable silicone rubber curable through condensation of silicon-bonded hydroxyls with silicon-bonded alkoxy radicals is improved by contacting the metal surface with aqueous solutions of sodium ammonium or potassium salts of chlorides, iodides, bromides, bicarbonates, sulfates, dihydrogenphosphates, monohydrogenphosphates, tetraborates, perchlorates, nitrates or nitrites, rinsing the surface, drying, thereafter applying the room temperature vulcanizable silicone rubber to the treated surface and allowing it to cure.

United States Patent [1 1 Greenlee Aug. 28, 1973 [75] lnventor: ThomasW. Greenlee, Midland,

Mich.

[73] Assignee: Dow Corning Corporation, Midland,

Mich.

[22] Filed: June 18, 1971 [21] Appl. No.: 154,645

[52] U.S. Cl. 117/49, 117/132 BS [51] Int. Cl. B44d 1/40, 1332b 15/08[58] Field of Search 117/49, 132 ES;

[5 6] References Cited UNITED STATES PATENTS 3,591,432 7/1971 Vazirani117/49 X 3,166,444 1/1965 Ehren et a1. 134/3 2,607,739 8/1952 De Long134/41 X 3,065,194 1l/l962 Nitzsche et a1. 260/37 2,999,077 9/1961Nitzche et a1 260/18 3,127,363 3/1964 Nitzche et a1 260/18 1,954,7444/1934 Peterson et a1. 134/41 X 2,671,717 3/1954 Ferguson 134/2 X2,008,839 7/1935 Samson 134/3 UX 2,878,188 3/1959 Callahan 134/3 UX2,458,661 l/1949 Webster et a1 134/3 OTHER PUBLICATIONS Burns et al.,Protective Coatings for Metals, pp. 22-25, 424, 425 & 427.

Primary Examiner-William D. Martin Assistant Examiner-Harry J. GwinnellAttorney-Robert F. Fleming, Jr. et a1.

[5 7] ABSTRACT The adhesion to metal surfaces of room temperaturevulcanizable silicone rubber curable through condensation ofsilicon-bonded hydroxyls with silicon-bonded alkoxy radicals is improvedby contacting the metal surface with aqueous solutions of sodiumammonium or potassium salts of chlorides, iodides, bromides,bicarbonates, sulfates, dihydrogenphosphates, monohydrogenphosphates,tetraborates, perchlorates, nitrates or nitrites, rinsing the surface,drying, thereafter applying the room temperature vulcanizable siliconerubber to the treated surface and allowing it to cure.

10 Claims, No Drawings METHOD OF ADHERING SILICONE RUBBER TO METALSURFACES USING SALTS This invention relates to a method of improving theadhesion of a room temperature vulcanizable silicone rubber to a metalsurface.

The adhesion of silicone rubber to metal surfaces and particularlycertain silicone rubbers to metal surfaces is a problem which hasreceived considerable attention in the prior art. In 1944, a method wasdescribed in U.S. Pat. No. 2,354,01 l which applied a layer ofinsolubleheavy metal salts of aliphatic fatty acids to metal surfaces to which anorganic rubber was to be bonded. Metal salts of fatty acids usually actas release agents for silicone rubber and thus a number of other methodsfor adhering silicone rubber to metals have been developed. The methodsknown in the art to adhere silicone rubber to metal surfaces involve theapplication of an adhesive composition, a primer and the like, such thatthe silicone rubber bonds to another surface instead of a metal surface.

An object of the present invention is to bond silicone rubber,particularly room temperature vulcanizable, to metal surfaces withoutusing an intervening layer of primer or adhesive. This object and otherswill become more apparent from the following detailed description of thepresent invention.

This invention relates to a method of improving the adhesion of roomtemperature vulcanizable silicone rubber to a metal surface comprisingcontacting a metal surface with an aqueous salt solution wherein thecation is selected from the group consisting of sodium and potassium andthe anion is selected from the group consisting of chloride, bromide,iodide, bicarbonate, sulfate, dihydrogenphosphate,monohydrogenphosphate, tetraborate, perchlorate, nitrate and nitrite,rinsing the metal surface with water, drying the metal surface wherein atreated metal surface is obtained, applying to the treated metal surfacea room temperature vulcanizable silicone rubber consisting essentiallyof a hydroxyl endblocked polydiorganosiloxane having a viscosity offrom1,000 to 100,000 cs. at 25C. and having organic radicals selected fromthe group consisting of methyl, ethyl, phenyl and 3,3,3-trifluropropyl,wherein at least 50 percent of the organic radicals are methyl radicals,an alkoxysilicon compound crosslinking agent and a curing catalyst,allowing the room temperature vulcanizable silicone rubber to curewhereby the adhesion of the room temperature vulcanizable siliconerubber to the treated metal surface is increased compared to theadhesion of the same room temperature vulcanizable silicone rubberapplied to an untreated metal surface.

The adhesion of room temperature vulcanizable silicone rubber to a metalsurface is improved by first contacting the metal surface with anaqueous solution of a sodium, ammonium or potassium salt. These sodium,ammonium and potassium salts are well known in the art and are readilyavailable materials. The anions of these salts can be chloride, bromide,iodide, bicarbonate, sulfate, dihydrogenphosphate,monohydrogenphosphate, tetraborate, perchlorate, nitrate and nitrite.The salts include, sodium chloride, potassium chloride, sodium bromide,potassium bromide, sodium iodide, potassium iodide, sodium bicarbonate,potassium bicarbonate, sodium sulfate, potassium sulfate, sodiumdihydrogenphosphate, potassium dihydrogenphosphate, sodiummonohydrogenphosphate, potassium monohydrogenphosphate, sodiumtetraborate, potassium tetraborate, sodium perchlorate, potassiumperchlorate, sodium nitrate, potassium nitrate, sodium nitrate,potassium nitrite, ammonium chloride, ammonium bromide, ammonium sulfateand ammonium nitrate.

The metal surface can be metals such as iron, steel, stainless steel,aluminum, titanium, tin and alloys thereof.

The metal surface can be contacted by aqueous solutions of any of thesalts or mixtures of the salts. Any method of contacting the metalsurface with the salt can be used so long as an intimate contact isaccomplished. The metal surface is preferably immersed in the aqueoussalt solution. Other methods, which are suitable, include methods suchas allowing the aqueous salt solution to pass over the metal surface,wetting a sponge or cloth with the aqueous salt solution and contactingthe surface with the wet applicator and the like. The time of contactcan be short such as one minute to long periods of time such as 64 hoursor longer, prefer ably the contact time is from I to 24 hours. Thetemperature of the contacting step is not narrowly critical except thattemperatures above the boiling point of the aqueous salt solution is notrecommended. The temperature can conveniently be ambient temperatures.Under certain conditions solvents other than water may be suitable as acarrier for the salts, particularly the ammonium salts.

The concentration of the salt in the aqueous salt solution can varybroadly such as from 0.1 weight percent salt up to the saturationconcentration. Preferably, con venient aqueous salt solutions are from 1to 10 weight percent salt.

After the metal surface has been contacted with the aqueous saltsolution for a suitable time, the metal surface is rinsed with water toremove any salt remaining on the metal surface. The method of rinsingcan be immersion of the metal surface in water, water can be run overthe metal surface and the like. After the metal surface is rinsed withwater, the surface is dried. The drying can be done by merely allowingthe metal surface to be exposed to ambient air. Other drying atmospherescan also be used such as nitrogen, a vacuum or the like. The drying canbe accomplished faster by rinsing away the water with a volatile solventsuch as acetone or methanol. Exposure of the metal surface to a dryingatmosphcre for periods of upto 10 days does not effect the ultimateadhesion of the room temperature vulcanizable silicone rubber to themetal surface.

To the treated metal surface, a room temperature vulcanizable siliconerubber is applied. The particular room temperature vulcanizable siliconerubber is limited to one which cures through the condensation of alkoxysbonded to silicone and hydroxyls bonded to silicon in the presence of acuring catalyst. These room temperature vulcanizable silicone rubbersare well known in the art and are available commercially. These roomtemperature vulcanizable silicone rubbers are known to possess a verylow affinity for bonding to metal surfaces and therefore it is importantto improve the adhesion to metal surfaces to broaden their utility. Theelimination of adhesion compositions and primers can also beaccomplished by treating the metal surface in accordance with thisinvention. Although the improvement in adhesion bond strength is notalways an ultimate value, completely cohesive, sufficient enhancement ofthe adhesive bond strength is realized to make these room temperaturevulcanizable silicone rubbers useful where adhesion to metal surfaces isneeded.

The room temperature vulcanizable silicone rubbers consist essentiallyof a hydroxyl endblocked polydiorganosiloxane having a viscosity of from1,000 to 100,000 cs. at 25C., preferably from 1,000 to 50,000 cs. at25C. The organic radicals of the polydiorganosiloxane can be methyl,ethyl, phenyl and 3,3,3- trifluoropropyl where at least 50 percent ofthe organic radicals are methyl radicals. The hydroxyl endblockedpolydiorganosiloxanes can be illustrated by polydimethylsiloxane,polymethyl-3,3,3-trifluoropropylsiloxane, polyethylmethylsiloxane,polydiethylsiloxane and polydiorganosiloxanes which are copolymers oftwo or more units of dimethylsiloxane units, diphenylsiloxane units,methylphenylsiloxane units, diethylsiloxane units and3,3,3-trifluoropropylmethylsiloxane units. Mixtures and blends ofpolydiorganosiloxanes can also be used. 1

The crosslinking agent can be an alkoxysilicon compound which can beillustrated by orthosilicates such as ethylorthosilicate,n-propylorthosilicate, amylorthosilicate, pentenylorthosilicate,diethyldipropylorthosilicate and hexylorthosilicate; alkylpolysilicatessuch as methylpolysilicate, ethylpolysilicate, isopropylpolysilicate,n-propylpolysilicate, secondary amylpolysilicate andn-butylpolysilicate; mono-organotrialkoxysilanes such asmethyltrimethoxysilane, ethyltriethoxysilane, propyltrimethoxysilane,methyltriisopropoxysilane, phenyltrimethoxysilane,vinyltrimethoxysilane; and other alkoxysilicon compounds such as Si(OCHC- H OCH Si(OCH CH OC1-1 CH C 11 Si(OC1-1 C- H OCH CF C1-1 C1'1 Si(OC1-1CH OCH and the like.

The curing catalyst can be illustrated by metal salts of carboxylicacids such as lead naphthenate, cobalt naphthenate, zinc naphthenate,sodium naphthenate, iron-2-ethylhexoate, lead 2- ethylhexoate, chromiumoctoate, dibutyltin dibenzoate, dibutyltin adipate, lead sebacate,dibutyltin dilactate, dibutyltindiacetate, dibutyltindilaurate, leadoctoate, tin ricinoleate, cobalt hexoate, aluminum acetyl-acetonate,zirconium acetoacetate, dioctyltindimaleinate, stannous octoate and thelike.

The above room temperature vulcanizable silicone rubbers are well knownin the art and are available commercially. The room temperaturevulcanizable silicone rubbers which comprise a hydroxyl endblockedpolydiorganosiloxane, an alkoxysilicon compound and a curing catalystare further defined in the following patents with respect toingredients, amounts and methods of preparation. The following patentsare hereby incorporated by reference and include U.S. Pat. No. 2,843,555by Berridge, U.S. Pat. No. 2,902,467 by Chipman, U.S. Pat. No. 2,927,907by Polmanteer, U.S. Pat. No. 3,065,194 by Nitzsche et al., U.S. Pat. No.3,070,559 by Nitzsche et al., U.S. Pat. No. 3,070,566 by Nitzsche etal., U.S. Pat. No. 3,127,363 by Nitzsche et al. and U.S. Pat. No.3,305,502 by Lampe. The room temperature vulcanizable silicone rubberscan also contain any of the fillers and additives which are well knownand described in the prior art.

After the room temperature vulcanizable silicone rubber is applied tothe treated metal surface, it is allowed to cure. Since these siliconerubbers vulcanize EXAMPLE 1 Aluminum panels and foil were initiallycleaned by immersing and rubbing with paper wipers first undertrichloroethylene and then under methyl isobutyl ketone. The cleanedaluminum substrates were allowed to dry and then immersed in thetreating agent for the 16 hours and then rinsed with water. The variousaqueous salt solutions used as treating agents were as defined in theTable. The rinsed aluminum substrates were allowed to dry under ambientconditions. The room temperature vulcanizable silicone rubber wasapplied with a spatula to the treated aluminum foil strips measuring 10inches by 1 inch by 0.004 inch. These foil strips were applied to atreated aluminum panel (6 inches by 4 inches by 1/16 inch) having a 4inch width with the rubber side of the foil strip to the aluminum paneland then rolled with a 10 pound hand roller to provide a thickness of 5to 8 mils for the room temperature vulcanizable silicone rubber. Thetest assembly was then allowed to cure at ambient conditions 1 weekbefore adhesion tests were made. Adhesion tests were also done onidentical samples after curing for 1 month. The adhesion was determinedby using conventional test equipment for the 180 peel test at 2 inchesper minute jaw separation rate. The adhesion results were as shown inthe Table.

The room temperature vulcanizable silicone rubber was prepared bythoroughly mixing the following ingredients: parts by weight of ahydroxyl endblocked polydimethylsiloxane fluid having a viscosity ofabout 12,500 cs. at 25C., 30 parts by weight of a trimethylsiloxytreated fume silica tiller, 4 parts by weight of npropylorthosilicateand 03 part by weight of dibutyltindiacetate.

TABLE Adhesion, Peel, Pounds Per Linear Inch Run No. Treatment AfterAfter 1 week 1 month 1. None (Control)* 2-5 2-6 2. Water 6 7 3. 2 weightpercent NaHCO, in water 7 14" 4. 2 weight percent Na SO in water 9 22 5.1 weight percent NaH,PO -H,O and 15 17 1 weight percent Na,HPO in water6. 2 weight percent Na,B,O,-10 H 0 21 25 in water 7. 1 weight percent1(C10 in water 9 15 8. 2 weight percent NaCl(0.342 M) 11 19 in water 9.2 weight percent KBr in water 16 17 10. 2 weight percent K1 in water 1822 1 l. 2 weight percent KNO, in water 20 19 12. 2 weight percent NaNO,in water 17 22 13. NaCl and Nal (0.342 M each) in water 5 13 14. KNO;and Na,SO (0.198 M each) in water 5 10 For comparative purposes 19 pli,after 8 months That which is claimed is:

1. A method of improving the adhesion of room temperature vulcanizablesilicone rubber to a metal surface consisting essentially of contactinga cleaned metal surface wherein said metal is selected from the groupconsisting of iron, steel, stainless steel, aluminum, titanium, tin andalloys thereof with an aqueous salt solution wherein the cation isselected from the group consisting of sodium, ammonium and potassium andthe anion is selected from the group consisting of chloride, bromide,iodide, bicarbonate, sulfate, dihydrogenphosphate,monohydrogenphosphate, tetraborate, perchlorate, nitrate and nitrite,rinsing the metal surface with water, drying the metal surface wherein atreated metal surface is obtained, applying to the treated metal surfacea room temperature vulcanizable silicone rubber consisting essentiallyof a hydroxyl endblocked polydiorganosiloxane having a viscosity of from1,000 to 100,000 cs. at 25C. and having organic radicals selected fromthe group consisting of methyl, ethyl, phenyl and 3,3,3-trifluoropropyl,wherein at least 50 percent of the organic radicals are methyl radicals,an alkoxy silicon compound crosslinking agent and a curing catalyst,allowing the room temperature vulcanizable silicone rubber to curewhereby the adhesion of the room temperature vulcanizable siliconerubber to the treated metal surface is increased compared to theadhesion of the same room temperature vulcanizable silicone rubberapplied to an untreated metal surface.

2. The method in accordance with claim 1 wherein the salt is a sodiumsalt.

3. The method in accordance with claim 1 wherein the salt is a potassiumsalt.

4. The method in accordance with claim 2 wherein the sodium salt issodium tetraborate.

5. The method in accordance with claim 2 wherein the sodium salt is asodium nitrite.

6. The method in accordance with claim 2 wherein the sodium salt issodium chloride.

7. The method in accordance with claim 2 wherein the sodium salt is amixture of sodium dihydrogenphosphate and sodium monohydrogenphosphate.

8. The method in accordance with claim 3 wherein the potassium salt ispotassium bromide.

9. The method in accordance with claim 3 wherein the potassium salt ispotassium iodide.

10. The method in accordance with claim 3 wherein the potassium salt ispotassium nitrate.

2. The method in accordance with claim 1 wherein the salt is a sodiumsalt.
 3. The method in accordance with claim 1 wherein the salt is apotassium salt.
 4. The method in accordance with claim 2 wherein thesodium salt is sodium tetraborate.
 5. The method in accordance withclaim 2 wherein the sodium salt is a sodium nitrite.
 6. The method inaccordance with claim 2 wherein the sodium salt is sodium chloride. 7.The method in accordance with claim 2 wherein the sodium salt is amixture of sodium dihydrogenphosphate and sodium monohydrogenphosphate.8. The method in accordance with claim 3 wherein the potassium salt ispotassium bromide.
 9. The method in accordance with claim 3 wherein thepotassium salt is potassium iodide.
 10. The method in accordance withclaim 3 wherein the potassium salt is potassium nitrate.